The present invention relates generally to voltage scale-down circuits, and more specifically, to a voltage scale-down circuit for Universal Serial Bus input/output (I/O) circuits.
Universal Serial Bus (USB) is an industry protocol that defines standards for cables, connectors, and communication protocols used in bus connections, communication, and power supply between computers and electronic devices. USB was developed to standardize connection of peripheral devices such as a mouse, keyboards, printers, and network devices to computing devices. The high reliability of USB ports has led to an increase in their popularity, making them a commonly used interface for smart phones and other electronic gadgets.
USB standard devices intended for supporting full-speed devices such as disk-drives are supposed to support a full-speed 5 Gbps (Gigabits per second). USB ports receive input signals that range from 0 to 3.3V with a common mode range of 0.8V to 2.5V. The unavailability of high-voltage devices (e.g., 3.3 V) for low-scale technologies, e.g., a 40 nanometer (nm) technology, forces a system designer to use low-voltage (e.g., 1.8 V) devices to design the 3.3 V USB I/O circuit. The low-voltage devices become less reliable when used in high-voltage circuits, primarily due to the over-stressing of transistor gate, source, and drain terminals.
To increase reliability of low-voltage devices used in high-voltage circuits, scale-down circuits are provided for scaling down the input signals from a high voltage level to a low voltage level. A resistor-ladder including series connected resistors is commonly used as a scale-down circuit. However, the resistor-ladder has several disadvantages. USB 1.1 and above define a minimum input impedance of 300 kilo-ohms (kΩ). However, using a 300 kΩ resistor at input terminals of the USB I/O circuit introduces a large area overhead. The USB I/O circuit coupled with the resistor-ladder also suffers a bandwidth limitation due to high resistance. The input signal swing is attenuated to half due to the resistor-ladder.
It would be advantageous to have a voltage scale-down circuit that ensures linear operation across the common mode range, reduces area overhead, and overcomes the above-mentioned limitations of conventional voltage scale-down circuits.